1. Field of the Invention
The present invention relates to reformed metal beverage closures and more particularly, to increased strength metal beverage closures within present industry standard dimensional specifications.
2. Description of the Prior Art
The prior art discloses numerous examples of metal closures for use with beverage containers. The majority of such closures includes a substantially planar center panel, a countersink portion around such center panel bounded on the inside by an integral inner panel wall and on the outside by an integral chuckwall, a first curved portion integrally joining said inner panel wall to said center panel, a second curved portion at the bottom of said countersink and integrally joining said chuckwall to said inner panel wall, and a peripheral flange extending radially outward from said chuckwall for attachment of the closure to the beverage container. Such containers also typically have an opening panel located on said planar center panel through which the contents of the container are accessed.
Until recently, the material structural dimensions of such closures were typically formed at a single working station concurrently with punching the closure from a metal blank. Closures are then placed through a multistep conversion process during which the opening panel is provided. Recently, in attempts to increase the strength of such closures, there has been considerable experimentation in providing a subsequent alteration to the material structural dimensions of the closure during the conversion process to provide increased strength. One example of such efforts is set forth in U.S. Pat. No. 4,031,837 which relates to altering standard closures in the conversion process by reforming and thereby reducing the radius of the second curved portion at the bottom of the countersink portion of the closure. As taught therein, the provision of a reduced radius in said second curved portion of a closure results in a significant increase in strength and a substantially perpendicular inner panel wall will add to such strength. Experimentation with the teachings of U.S. Pat. No. 4,031,837 has indicated that when a closure is so reformed, the countersink depth decreases while the panel height increases causing the product to be out of industry standards, which makes the product not interchangeable with other suppliers' product, necessitates new seaming tooling by the customer, and creates tab over problems at low internal pressure. The new seaming tooling required by such closures in them is not useable on standard closures. Another approach was tried by forming a nonstandard shell with standard tooling, then utilizing tooling as taught to reform the shell to standard dimensions in U.S. Pat. No. 4,031,837. Results showed that it is extremely difficult or impossible to obtain the type of constant countersink radius provided to closures initially. That is, the radius tends to begin at the juncture of the chuckwall and the second curved portion at the desired reduced radius and then the radius gradually increases until it is back to standard at the juncture of the second curved portion with the inner panel wall. The inner panel wall will also be deformed after the reform to include two straight portions with an additional radius. Although closures produced in conformance with the teachings of U.S. Pat. No. 4,031,837 show a significant increase in strength, in addition to requiring different tooling by customers, the varying radius and deformed inner panel wall detract from the potential strength realizable from a constant reduced radius and straight inner panel wall with a substantially perpendicular orientation. Attempts to realize this potential by providing a reduced radius in the initial forming step have uniformly been unsuccessful as the severe working which the closure undergoes results in a high incidence of fracture and other defects.
Other efforts at increasing strength during the conversion process include tension doming of the center panel portion and coining the annular segment of the closure which comprises the curved portion attaching the center panel to the inner panel wall for the purpose of work hardening and stiffening this segment, both as taught in U.S. Pat. No. 4,217,843.
The goal of the above-described efforts is to provide a reduced gauge sheet metal closure which has the required strength to resist buckling of the closure at internal pressures of 85 and 90 pounds per square inch respectively for soft drink and beer closures. In addition, it is necessary that the closure exhibit a commensurate rock resistance to standard gauge closures as discussed in U.S. Pat. No. 4,217,843. Briefly, rock resistance is defined as the pressure at which the tab or other opening means located on the center panel will be forced above the rim of the beverage container which then exposes the tab to accidental and inadvertent opening when being transported on conveyors or otherwise. As discussed in U.S. Pat. No. 4,217,843, this is one of the major drawbacks of proposals such as that disclosed in U.S. Pat. No. 4,031,837, which contemplate increasing the depth of the center panel with regard to the lower edge of the outer countersink above present standard dimensions. Although U.S. Pat. No. 4,217,843 improves on the prior art, it does so by increasing panel height which places the tab closer to the rim of the closure.
It should also be appreciated that innovations in the metal beverage container industry are strictly limited in scope as articles produced must be compatible with existing customer handling equipment. Most large purchasers of beverage containers utilize more than a single source of supply, and it is therefore necessary that any innovations be made within the specifications set by the customer. Innovations not meeting this criteria will, at least presently, not be commercially acceptable.